Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people, just like you, are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions

Calculate Screen Coordinates (Determine Screen Coordinates from World Coordinates of an object)

Posted on 2004-07-31
Last Modified: 2013-12-26
The Problem is: I can't fully understand this function wich calculates the screen coordinates of an given object in world coordinates:

inline void Calculate_Screen_Coordinates(float* pScreenX, float* pScreenY, D3DXVECTOR3* pVec)
    tempX = pVec->x;
    tempY = pVec->y;
    tempZ = pVec->z;

    tempX2 = ViewProjectionMatrix._11*tempX + ViewProjectionMatrix._21*tempY + ViewProjectionMatrix._31*tempZ + ViewProjectionMatrix._41;
    tempY2 = ViewProjectionMatrix._12*tempX + ViewProjectionMatrix._22*tempY + ViewProjectionMatrix._32*tempZ + ViewProjectionMatrix._42;
    tempW2 = ViewProjectionMatrix._14*tempX + ViewProjectionMatrix._24*tempY + ViewProjectionMatrix._34*tempZ + ViewProjectionMatrix._44;

// Here comes part wich i can't understand fully:
     tempInvW2 = 1.0f/tempW2;

    *pScreenX = (1.0f + (tempX2*tempInvW2))*0.5f*screenwidth;
    *pScreenY = (1.0f - (tempY2*tempInvW2))*0.5f*screenheight;  

In detail:
// Next line i can't understand ->
tempInvW2 = 1.0f/tempW2;
i only know that this is a faktor for the z coord because the screen has only x/y but i don't know
why 1.0f and why the variable name is tempInv like Invers ?

This line is clearly:
*pScreenX = (1.0f + (tempX2*tempInvW2))*0.5f*screenwidth;

This line seems me wrong:
*pScreenY = (1.0f - (tempY2*tempInvW2))*0.5f*screenheight;
==> i would say:
*pScreenY = (1.0f + (tempY2*tempInvW
because the viewport coordinates are from -1/-1 to 1/1 and the screen coords are beginning in the  left top corner at 0/0 therefore transform -1/-1 & 1/1 to 0/0 & 2/2 ?!

Can anybody explain this in detail ?
Question by:WolfgangKoenig
LVL 17

Accepted Solution

davebytes earned 125 total points
ID: 11743364
A similar question/explanation at:

W is the scaling factor for the viewport.  Dividing by it will get you into screen-relative space.  1.0/W is pre-calculating the inverse so that a multiply can be used -- it is only used twice, but still faster to calculate the inverseW once and multiply against it, rather than dividing twice (yes, division is THAT BAD).

The - is because the 0,0 in 3D screen coords is usually the BOTTOM left, need to flip Y to get it to the TOP left.

As you say, generated view coordinates will be in the range of [-1,1] so for x:
1 + [-1,1] => [0,2] * 0.5 => [0,1]
and for y:
1 - [-1,1] => [2,0] * 0.5 => [1,0]
thus flipping the Y axis on the screen properly.

Multiplying by height/width then gets into screen pixels.


Featured Post

Announcing the Most Valuable Experts of 2016

MVEs are more concerned with the satisfaction of those they help than with the considerable points they can earn. They are the types of people you feel privileged to call colleagues. Join us in honoring this amazing group of Experts.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

What is RenderMan: RenderMan is a not any particular piece of software. RenderMan is an industry standard, defining set of rules that any rendering software should use, to be RenderMan-compliant. Pixar's RenderMan is a flagship implementation of …
As game developers, we quickly learn that Artificial Intelligence (AI) doesn’t need to be so tough.  To reference Space Ghost: “Moltar, I have a giant brain that is able to reduce any complex machine into a simple yes or no answer. (http://www.youtu…
A short tutorial showing how to set up an email signature in Outlook on the Web (previously known as OWA). For free email signatures designs, visit https://www.mail-signatures.com/articles/signature-templates/?sts=6651 If you want to manage em…
Finds all prime numbers in a range requested and places them in a public primes() array. I've demostrated a template size of 30 (2 * 3 * 5) but larger templates can be built such 210  (2 * 3 * 5 * 7) or 2310  (2 * 3 * 5 * 7 * 11). The larger templa…

839 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question